1. Field of the Invention
This invention relates to semiconductor package Surface Mount Technology (“SMT”) and, particularly, to surface mount of packages having an exposed die paddle surface.
2. Description of Related Art
A conventional leadframe semiconductor die package has a semiconductor die mounted onto and electrically connected to a leadframe. The leadframe, which may be formed by patterning a sheet of electrically conductive metal such as copper, typically includes a die paddle, onto which the die is affixed, and leads, to which the die is electrically connected. The mounted die may be encapsulated or molded, and the leads may project from one or more sidewalls of the molding or encapsulation (a “leaded package”); or the leads may end at one or more of the sidewalls of the molding or encapsulation (a “leadless package” or “no-lead package”).
An encapsulated leadless package is shaped generally as a thin rectangular parallepiped having larger rectangular or square upper and lower sides. Typically, lower surfaces of the die paddle and leads are exposed at the lower (package mount) side of the package, and end surfaces of the leads are exposed at (and are generally flush with) one or more sidewalls of the package. Thus a typical leadless semiconductor die package appears on the package mount side as a thin rectangular or square solid block of the molding or encapsulation material, with die paddle surfaces exposed flush with the “underside” (lower or package mount side) of the package and leads exposed flush with the underside and with one or more of the sidewalls of the package adjacent the lower edges. A quad flat no lead (“QFN”) package has leads exposed flush with the sidewalls adjacent all four lower edges, and a dual flat no lead (“DFN”) package has leads exposed flush with the sidewalls adjacent two (typically opposite) lower edges.
Various approaches to mounting the die in the package, and electrically connecting the die to the leadframe, are in use. Generally the die may be connected by wire bonds or by flip-chip interconnection, for example.
In a conventional flip chip package, electrically conductive balls or bumps are affixed to the bond pads on the die, and the die is mounted “die-down”, that is, the die is oriented so that the active side of the die faces toward the leadframe. The balls or bumps are aligned with bond sites on the die paddle to effect electrical connection of the die to the leadframe.
In a conventional wire-bonded leadframe package the die is mounted “die-up”, that is, the die is oriented so that the active side of the die faces away from the leadframe. In such a package the die may be affixed to the die mount surface of the die paddle using a die attach adhesive, and the die is electrically connected to the leadframe by wires connecting interconnect pads on the die with bond sites on the leads (and, in some instances, with bond sites on the die paddle).
The functioning die can generate a significant amount of heat. In some configurations, particularly where the die paddle is exposed at the back side of the package, the die paddle may carry heat from the die away from the package to the underlying substrate. For example, the die may be affixed to the die paddle using a thermally conductive (and, optionally, electrically insulative) adhesive, so that heat is conducted from the back side of the die, through the adhesive, to the die paddle.
A flat no-lead package may be installed by surface mount onto a support such as a printed circuit board. Bond fingers are exposed at the package mount surface of the support to provide electrical connection of the package to circuitry in the support. In a conventional surface mount, the bond fingers are situated so that the exposed leads at the back side of the package can be aligned with corresponding bond fingers. An electrically conductive material, typically a solder paste, is applied onto the bond fingers on the support, and the package is mounted by placing the package in alignment onto the support and applying heat to reflow the solder and complete the connection. In package configurations having an exposed die paddle, the support may additionally have an exposed thermal pad situated in alignment with the die paddle; a solder paste may be provided at the thermal pad as well as at the bond fingers, to provide good thermal conduction from the package to the support. Typically thermally conductive vias carry heat from the thermal pad to the opposite side of the support, or to a thermally conductive layer (such as a ground plane) in the support.
The package is typically subjected to heat stress, for example by thermal cycling, and is then visually inspected and electrically tested before it is mounted on the support. Packages that fail visual inspection or electrical testing are discarded.